August 2019

IN FOCUS

Cataract Surgery Post-LVC
IOL power calculation after refractive surgery


by Chiles Aedam Samaniego EyeWorld Contributing Writer

Any IOL power calculation has three key ingredients, according to Uday Devgan, MD: (1) the power of the cornea, (2) the axial length of the eye, and (3) the effective lens position (ELP).
This applies to all eyes, whether or not their corneas had previously undergone surgery, such that Zaina Al-Mohtaseb, MD, said that the principles for achieving good refractive outcomes “are the same as for virgin corneas as patients who have received prior corneal procedures, which include accurate biometry and keratometry, optimizing IOL calculations, and ruling out ocular pathology”—particularly corneal and retinal conditions such as ocular surface disease, epithelial basement membrane dystrophy (EBMD), and epiretinal membranes (ERM).
However, prior corneal refractive procedures definitely influence outcomes, affecting the accuracy of IOL power calculations. EyeWorld consulted Dr. Al-Mohtaseb and Dr. Devgan, as well as Damien Gatinel, MD, and Douglas Koch, MD, on improving the accuracy of IOL power calculations in post-refractive surgery patients.

Post-refractive challenges

According to Dr. Al-Mohtaseb, IOL power calculations become more challenging primarily for two reasons: (1) inaccurate calculation of total corneal refractive power due to the ratio change between the anterior and posterior corneal curvature, and (2) the increased difficulty of predicting ELP.
Regarding the first problem, Dr. Devgan noted that “when we change the front curvature of the cornea with LASIK but not the back curvature, the biometer does not read as accurately.”
On the second problem, Dr. Devgan explained, “With prior corneal refractive surgery, like LASIK, we have significant changes to the corneal power. Since the IOL power formulae tend to use the corneal power to determine the ELP, this is also thrown off. We can minimize this by using the Aramberri double-K method, which uses the measured keratometry for the vergence calculation but a more normal K value for the ELP determination. We can also use newer formulae that will place more emphasis on a measured anterior chamber depth.”
Dr. Gatinel also pointed out how predicting ELP loses accuracy in post-refractive surgery eyes, complicating IOL power calculations, but added that other factors affecting IOL power calculations include tear film changes altering topography readings, irregular astigmatism, subtle refractive index changes from epithelial hyperplasia, biomechanical instability following radial keratotomy, and some “peculiar effects” cataract surgery may have on corneal wall geometry and hydration.
There are many factors to consider, and physicians do not yet have a perfect understanding of how everything fits together. “This is an issue that hasn’t been thoroughly worked out; factors such as altered higher order aberrations have yet to be understood and incorporated,” Dr. Koch said.

Role of diagnostics

Dr. Gatinel noted that the accuracy and precision of IOL power calculations depend on the quality of preop measurements. He said that accessing epithelial and posterior corneal mapping could, in theory, improve the corneal power estimation. Optical biometry has also “tremendously improved” in accuracy, and 2-D and eventually 3-D reconstructions of the anterior segment and even the whole eye “may bring another level of precision and enable some fancy ray tracing techniques,” he said.
Dr. Al-Mohtaseb is optimistic about developments in diagnostics and the effect they will have on IOL power calculations. “The accuracy of biometry and topography are continuing to improve. As we get more accurate measurements of total corneal power even in post-refractive eyes, we will have an increase in accuracy of IOL power calculations,” she said.
Dr. Koch is more cautious, highlighting how imperfectly all the factors affecting IOL power calculations are currently understood.
“I think that the most promising option is improved anterior and posterior corneal topographic measurements, combined with ray tracing,” he said.
To predict corneal power, “we have to rely on corneal topography, and we’re hoping that eventually we’ll get a meaningful posterior cornea measurement,” he added. “I am cautiously encouraged by some data we have generated from the IOLMaster 700 [Carl Zeiss Meditec] that shows the total keratometry value could be an improved measure of corneal power compared to simply measuring the anterior cornea and guessing posterior corneal power.”
Dr. Koch cited their study in which they used total keratometry from the IOLMaster 700 with the standard Haigis formula. They found the combination to be as accurate as formulas that were specifically designed to be used in post-LASIK eyes.1 “That said, the overall results still weren’t stellar, so we still have a long way to go,” he cautioned.
“There is no one magical device that gives us a perfect measurement every time. At this point our best bet is to use all the biometers and see if they tend to agree,” Dr. Devgan said. “Also, we must emphasize to patients that particularly in a post-LASIK eye, we are doing IOL power estimations and not exact calculations.” The final outcome is also affected by the patient’s anatomy and healing response, he added.

Meeting the challenge

“Numerous strategies have been developed to account for the challenges of post-refractive surgery IOL calculations,” Dr. Al-Mohtaseb said. She cited the ASCRS website, which provides current IOL power calculation formulas that have demonstrated effectiveness in post-LASIK eyes (ascrs.org). “There is no single formula that has been shown to be demonstrably better in all cases,” she added. “Of the available methods using no pre-refractive data, the Holladay 2, OCT IOL power formula, Barrett True K formula, and intraoperative aberrometry show promising results in comparative studies. I like using those three preoperative formulas in addition to the intraoperative aberrometry measurement before I make the final IOL decision.”
“With more than 20 different methods of IOL power estimation in post-refractive eyes, we know that none of them must be truly accurate across the board,” Dr. Devgan said. “My preference is to use the ASCRS online post-refractive calculator and enter all of the data available to me. This calculator computes all possible formulae for which data has been supplied. This will give a range of values, most of which tend to agree. This agreement among multiple formulae is comforting but still not a guarantee of accuracy. Remember that in all eyes, the patient’s healing response will affect the ELP and the refractive outcome.”  
Of the current formulas, Dr. Koch favors the Barrett True K, the Haigis-L, and the OCT-based formula, available on ascrs.org. He also uses the Masket when prior refractive history is not available.
A particular challenge that can arise is a discrepancy between biometry and topographic keratometry measurements. “If there is a difference between biometry and topo Ks after optimizing the surface and making sure that the measurement quality is good, it is most likely due to the different location of measurements on the different machines (3-mm vs. 1.6-mm zones, for example),” Dr. Al-Mohtaseb said. “In those cases if there is a large difference in measurements, I would avoid putting in a toric lens or correcting astigmatism. I would most likely lean toward my biometry measurement (LENSTAR [Haag-Streit] or IOLMaster 700).”
“Underlying dry eye syndrome or other ocular surface abnormality, such as EBMD, should be treated and optimized first,” Dr. Devgan said. “The corneal topography and tomography give us a better idea of what is happening in the entire cornea compared to simple keratometry, which just gives us average corneal powers and astigmatism. I prefer the topography and tomography for determination of regularity, symmetry, and axis of astigmatism but will give more weight to keratometry from the optical biometer for the IOL power estimation.”
Dr. Koch recommends looking for evidence of the biometry’s accuracy in the quality of the mires, the standard deviations of the magnitude and meridian, and the regularity of the cornea. “I will often remeasure, and for these cases will also use ORA [Alcon] in the operating room to help guide me, but there definitely are times that you don’t get them to match, and you have to use your best judgment,” he said. “It’s not straightforward.”
“I usually measure patients two or three different ways—LENSTAR, IOLMaster 700, Galilei [Ziemer Ophthalmic Systems]—and we also have an OCT formula—we use the Avanti OCT [Optovue],” he added. Dr. Koch tries to develop a consensus based on all the data, leaning toward clustered measurements. Still, “sometimes going with the cluster doesn’t get you the advantage you want,” citing a recent case in which the cluster including ORA was 1 D off and the OCT formula was the only one that was accurate.
Ultimately, he shows all patients—and those accompanying them—the variability of data from the ASCRS website calculations and explains that glasses, contact lenses, more corneal refractive surgery, or even an IOL exchange may be required to achieve the desired result.
“That’s just the reality of our imperfect science right now,” he said.
Dr. Devgan offered a final pearl: “When in doubt, choose the higher IOL power to err on the side of residual myopia, which provides a benefit for intermediate or near vision and if plano is desired, it is easy to treat with additional excimer laser ablation.”

At a glance

• Corneal power, axial length, and ELP are key ingredients of current IOL power calculations.
• Improving the accuracy of diagnostics improves the accuracy of power calculations, but a better understanding than physicians currently have of all the factors affecting the refractive power of the eye is also necessary.
• With no one device and IOL power calculation formula able to provide the best results in all cases, surgeons should use multiple devices and formulas along with their best judgment to optimize outcomes.

Contact information

Al-Mohtaseb
: zaina1225@gmail.com
Devgan: devgan@gmail.com
Gatinel: gatinel@gmail.com
Koch: dkoch@bcm.edu

About the doctors

Zaina Al-Mohtaseb, MD
Assistant professor of
ophthalmology
Baylor College of Medicine
Houston

Uday Devgan, MD
Chief of ophthalmology
Olive View UCLA Medical Center
Los Angeles

Damien Gatinel, MD
Head of the Anterior Segment and Refractive Surgery Department
Rothschild Foundation
Paris, France

Douglas Koch, MD
Professor and Allen, Mosbacher, and Law Chair in Ophthalmology
Cullen Eye Institute
Baylor College of Medicine
Houston

Reference

1. Wang L, et al. Evaluation of total keratometry and its accuracy for IOL power calculation in eyes following corneal refractive surgery. J Cataract Refract Surg. Article in press.

Financial interests

Al-Mohtaseb
: Alcon, Carl Zeiss Meditec, Johnson & Johnson Vision
Devgan: www.IOLcalc.com, Advanced Euclidean Solutions, CataractCoach.com
Gatinel: Alcon, Bausch + Lomb, Nidek, PhysIOL
Koch: Alcon, Carl Zeiss Meditec, Johnson & Johnson Vision

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